1. Field of the Invention
The invention pertains to the field of electrical measuring and testing. More particularly, the invention pertains to radio-frequency power measurement using directional couplers.
2. Description of Related Art
A form of directional coupler used in RF measurement is often described as a “Bruene Bridge” or “Bruene-type coupler”, after the article “An Inside Picture of Directional Wattmeters”, by Warren B Bruene, W5OLY, published in QST magazine on April 1959, p 24-28.
FIG. 8 shows Bruene's drawing of a directional wattmeter circuit from his 1959 QST article (also FIG. 8 in the article, page 27). The wattmeter 300 is made up of a directional coupler 301 and a control head 320. The two components might be integrated into a single case, or, often, are set up with the coupler 301 remote from the control head 320, linked by wires 323 and 324, bearing voltages representative of reflected power and forward power, respectively. A switch 321 allows the meter 322 to read either forward or reflected power, as desired.
As shown in FIG. 8, the coupler 301 is inserted into the feedline between the output of a transmitter 302 and the line to an antenna 303. The feedline is shown here schematically as a coaxial cable, with the shield 304 and 305 being grounded, and the center conductor 307 running continuously from transmitter 302 to antenna 303 lines through a toroid 306. The RF current I in the center conductor 307 is sampled by inductive coupling to toroid coil 306. The current I induces a voltage in coil 308 wrapped around the toroid 307, and as a result current i flows through the coil 308 and its series resistors R. The value of resistance R is kept small compared with the coil 308 reactance so it has little effect on coil current. The coil current i then is determined by the induced voltage vI across resistors R, and the resistance of coil 308.
Detector diodes 309 and 310 rectify the voltage to ev on each side of the coil 308, and, through chokes 311 and 312 and calibrating resistors 313 and 314, feed wires 323 and 324 through switch 321 to allow meter 322 to read reflected and forward power, respectively.
A problem of prior art measurement approaches is that accurate radio frequency (RF) power measurements require careful and complementary design and construction of both the coupler that is inserted in line with the transmission line on which measurements are to be performed and the control head, which processes DC output signals from the coupler to determine the power present on the transmission line and display the results to a user.
At low power levels, such couplers are typically problematic because of the voltage drop and nonlinearity of diode detectors typically used in Bruene-type couplers. At high power levels, problems can arise due to saturation of the toroid core typically used in couplers. Furthermore, couplers are generally not flat across the frequency spectrum, meaning that their voltage-to-power curve (also referred to as its transfer function) typically varies, and is not equal level for a given incident power when the operating band (frequency) is changed, as is typically encountered when performing power measurements in Amateur Radio and other applications.